First, all of the 21.5-Inch "Late 2013" iMac models have Quad Core processors that are
much faster than the
Dual Core Intel Core i5 (I5-4260U) processor in the "Mid-2014" model.
Second, the 21.5-Inch "Late 2013" iMac models have much faster graphics processors -- either Iris Pro 5200 integrated graphics or GeForce GT 750M dedicated graphics -- whereas the "Mid-2014" model is equipped with older and slower Intel HD Graphics 5000.
Third, and perhaps most damning, it is needlessly inconvenient to
upgrade the RAM in the 21.5-Inch "Late 2013" iMac models, but they can be upgraded to 16 GB with some effort. By contrast, 8 GB of RAM is soldered in place in the "Mid-2014" iMac model and it
cannot be upgraded at all, whether at the time of purchase or later.